MAINTAINERS: Add KVM x86/xen maintainer list

Adding Paul as co-maintainer of Xen support to help ensure that things
don't fall through the cracks when I spend three months at a time
travelling...

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>